It is generally desirable to employ a base in a heat developable light-sensitive material in order to accelerate development by heat. In order to increase the stability of the light-sensitive material, the base must be present in the form of a precursor. The term "base precursor" as used herein means a compound which thermally decomposes and releases a basic component upon heating. In order to employ such a precursor in practical use, the precursor desirably fulfills two opposite properties, i.e., stability at normal temperature and rapid decomposability at the time of heating.
Base precursors which are heretofore known include, for example, a urea (as described in U.S. Pat. No. 2,732,299 and Belgian Pat. No. 625,554), a method using urea or urea and an ammonium salt of a weak acid (as described in Japanese Patent Publication No. 1699/65), a method using hexamethylenetetramine or semicarbazide (as described in U.S. Pat. No. 3,157,503), a method using a triazine compound and a carboxylic acid (as described in U.S. Pat. No. 3,493,374), a dicyandiamide derivative (as described in U.S. Pat. No. 3,271,155), an N-sulfonyl urea (as described in U.S. Pat. No. 3,420,665), an aminimide (as described in Research Disclosure, RD-15776 (1977)), and a salt of a thermally decomposable acid represented by trichloroacetate (as described in British Pat. No. 998,949), etc.
However, image forming materials using these compounds as base precursors inherently have serious defects. Specifically, these compounds do not fulfill the above described indispensable conditions, i.e., good stability during storage at normal temperature and rapid decomposition at the time of development processing. As a result, problems occur in that a high image density cannot be obtained or the signal to noise (S/N) ratio of the image is seriously decreased due to release of the base during storage.